1. Field
Various embodiments described herein relate to optical modulation apparatuses and optical modulation methods.
2. Description of the Related Art
With an increase in transmission traffic, demand has been growing recently for introduction of a next-generation optical transmission system having a transmission capacity exceeding the existing 40 gigabit per second (Gbps). When signal transmission speed is simply increased for realization of the mass transmission capacity, realization of electric signal circuits to be used is difficult. For example, degradation of optical transmission signals, such as spectral degradation caused by optical filters and signal degradation caused by chromatic dispersion and optical noise accumulation, occurs. Accordingly, an optical transmission system adopting a multi-level phase modulation having good spectrum efficiency, optical signal-to-noise ratio (OSNR) tolerance, and non-linear tolerance seems to be promising. For example, quadrature phase-shift keying (QPSK) for four-level phase modulation is available as the multi-level phase modulation.
An optical modulation apparatus that includes a return-to-zero (RZ) modulator and adopts RZ differential QPSK (RZ-DQPSK) modulation is one type of QPSK optical modulation apparatuses. The RZ-DQPSK modulation is expected as a modulation candidate adopted in the next-generation optical transmission system because it characteristically has high spectrum efficiency and yields a modulated optical signal of a narrow spectrum.
The RZ-DQPSK optical modulation apparatus generally has an I-arm for superposing a data signal on an in-phase (I) component of light emitted by a light source and a Q-arm for superposing another data signal on a quadrature-phase (Q) component of the light emitted by the light source. The signals resulting from superposition of the data signals on the light at the I-arm and the Q-arm are multiplexed to be a DQPSK modulation signal. The RZ modulator then performs RZ modulation on the DQPSK modulation signal to yield an optical signal modulated according to RZ-DQPSK modulation.
At this time, the signals obtained at the I-arm and the Q-arm may be out of phase because of a temperature change or an aging change, for example. More specifically, a delay difference may occur between the I-component and the Q-component of the light to be multiplexed. The delay difference may impair the optical signal resulting from the RZ-DQPSK modulation. As a result, transmission performance decreases in optical transmission apparatuses for transmitting the optical signals.
To avoid such a circumstance, a technique is studied for monitoring power of an optical signal output from an RZ modulator and adjusting delays of data signals input to an I-arm and a Q-arm based on the monitoring result. FIG. 18 illustrates a configuration of such an optical modulation apparatus for adjusting the delays of the data signals. The optical modulation apparatus includes a laser diode (hereinafter, abbreviated as an “LD”) 11 serving as a light source, a DQPSK modulator 12a, an RZ modulator 12b, drivers (hereinafter, abbreviated as “DRVs”) 13a-13c, and phase shifters 14a-14c. The optical modulation apparatus also includes an optical coupler 21, a photo detector (hereinafter, abbreviated as a “PD”) 22, a band-pass filter (hereinafter, abbreviated as a “BPF”) 23, a power monitor (hereinafter, abbreviated as a “MON”) 24, and a controller 30.
Light generated by the LD 11 is input to the DQPSK modulator 12a. An I-arm and a Q-arm of the DQPSK modulator 12a superpose data signals from the DRVs 13a and 13b on an I-component and a Q-component of the light, respectively. The I-component and the Q-component of the light having the data signals superposed thereon are multiplexed to be a DQPSK modulation signal. The RZ modulator 12b then performs RZ modulation on the DQPSK modulation signal. At this time, the RZ modulator 12b performs the RZ modulation on the DQPSK modulation signal using a clock signal CLK from the DRV 13c. 
The optical coupler 21 splits the optical signal resulting from the RZ modulation. The PD 22 then converts the split optical signal into an electric signal. The electric signal passes through the BPF 23, whereby the MON 24 monitors power at a specific band of the electric signal. The controller 30 adjusts amounts of phase shift (hereinafter, referred to as phase-shift amounts) set in the phase shifters 14a and 14b in accordance with the monitoring result provided by the MON 24 to decrease a delay difference between the signals yielded at the I-arm and the Q-arm. At the same time, the controller 30 adjusts a phase-shift amount set in the phase shifter 14c in accordance with the monitoring result provided by the MON 24. As described above, the optical modulation apparatus monitors the power of the RZ-modulated signal and shifts the delays of the data signals in accordance with the monitoring result, thereby being able to decrease the delay difference between two signals to be multiplexed in multi-level phase modulation.
Japanese Unexamined Patent Application Publication No. 2007-329886 is an example of related art.
However, the method for monitoring the power of the signal and adjusting the delay difference in accordance with the monitoring result is based on an assumption that the RZ modulation is performed on the DQPSK modulation signal. An optical modulation apparatus without the RZ modulator unfortunately has difficulty appropriately controlling the delay difference. More specifically, an optical modulation apparatus adopting, for example, non return-to-zero DQPSK (NRZ-DQPSK) modulation does not perform RZ modulation on a DQPSK modulation signal. Accordingly, such an optical modulation apparatus has difficulty appropriately controlling a delay difference even if it monitors power of the signal.
To concretely explain this problem, FIG. 17 illustrates a signal waveform resulting from NRZ-DQPSK modulation and RZ-DQPSK modulation for each delay difference. More specifically, FIG. 17 illustrates waveforms of optical signals resulting from the NRZ-DQPSK modulation and the RZ-DQPSK modulation when the delay difference is 0 picoseconds (ps), 4 ps, and 8 ps. In each graph of FIG. 17, the horizontal axis represents time, whereas the vertical axis represents power.
As illustrated in FIG. 17, signal information at an area “A” illustrated in the drawing is extracted from the optical signal resulting from the RZ-DQPSK modulation through pulse carving of the RZ modulator. Accordingly, the MON 24 illustrated in FIG. 16 monitors power of the signal at the area “A” illustrated in FIG. 17. Since the power of the signal at the area “A” illustrated in FIG. 17 decreases as the delay difference increase, the controller 30 illustrated in FIG. 16 sets the phase-shift amounts so that a maximum value is monitored. In this way, the delay difference can be decreased.
In contrast, pulse carving is not performed by the RZ modulator on the optical signal resulting from the NRZ-DQPSK modulation. Accordingly, an average of the power of the optical signal of the whole area illustrated in each graph of FIG. 17 is monitored. As a result, since the monitored signal power does not change even if the delay difference changes, the monitoring result does not reflect the delay difference.
That is, as illustrated in FIG. 18, the monitored output decreases as the delay difference increases regarding the RZ-DQPSK modulation. In contrast, regarding the NRZ-DQPSK modulation, the monitored output is substantially constant even if the delay difference increases. Thus, monitoring the power of the non-RZ-modulation signal is not useful in appropriately controlling the delay difference between the two signals multiplexed by the DQPSK modulator. Since the delay difference is not appropriately controlled, the NRZ-DQPSK optical modulation apparatus unfortunately has difficulty suppressing degradation of the optical signal.
A similar problem occurs when, for example, polarization multiplexing is adopted in optical transmission. In the polarization multiplexing, data signals are superposed on different polarized components before the polarized components are multiplexed. A delay difference between the polarized components to be multiplexed degrades the multiplexed optical signal. Since the delay difference between the polarized components is not reflected in power of the multiplexed optical signal, it is difficult to suppress degradation of the optical signal by monitoring the power of the optical signal.